1. Field of the Invention
The present invention relates to a digital/analog converter and, more particularly, to an offset compensating apparatus of a digital/analog converter for compensating an offset generated from an LCD panel driving unit of a liquid crystal display (LCD) projection TV, and an offset compensating method using the same.
2. Description of the Related Art
Recently, the LCD projection TV employing an enlarged projection method is widely used as a large scale display device. The LCD projection TV uses a principle that as a strong light beam radiated from a lamp passes through an LCD, a TV image displayed on the LCD is projected on a screen. For the purpose of driving an LCD panel, the LCD projection TV includes a driving unit consisting of a digital/analog converter and a buffer unit.
However, in the LCD projection TV, an output value of the driving unit mainly contains an offset due to an influence of a peripheral circuit or a temperature. Since the offset affects considerably a picture quality of the screen, a laser trimming method has been adopted to remove the offset generated from the driving unit.
The laser trimming method is a technique for varying a resistance by trimming a resistance value with laser as mentioned in AD 8380 of Analog Device Co.
FIG. 1 is a circuit diagram of an offset compensating apparatus of a DAC trimming a resistance value by using laser in an LCD projection TV in accordance with a conventional art.
As shown in FIG. 1, in the conventional offset compensating apparatus, ‘0’ or ‘1’ is inputted as a switching signal as a digital input value from the least significant bit (LSB) to the most significant bit (MSB) in response to an analog output voltage (Vout) of a buffer unit 11 including an operational amplifier (OP) Amp, to turn on or off a switch to control the output voltage (Vout) of the OP Amp.
That is, in controlling the output voltage, if an offset occurs due to disagreement of resistances, resistance values of 2R and R are varied to change a value of ‘I′’ thereby obtaining an accurate value of the output voltage (Vout) of the OP Amp by using the laser trimming method, which can be expressed by the below equation (1):Vout=Vcom±I′×2R11  (1)wherein Vcom indicates an input voltage of a positive terminal of the OP Amp, 2R11 indicates a feedback resistance of the OP Amp, and I′ indicates a current flowing at the feedback resistance.
However, the resistance varying using the laser trimming is disadvantageous in that a chip unit price increases and a high-priced process expense is incurred to construct it.